Desired device dimensions and pitches have decreased to the point where traditional single patterning photolithographic techniques (e.g., 193 nm wavelength photolithography) cannot form a single patterned mask layer with all of the features of the final target pattern. Thus, device designers and manufacturers have begun utilizing various double patterning techniques requiring multiple exposures to define a single target pattern. Two such techniques are lithography-etch-lithography-etch (LELE) and SADP. The two techniques are similar in the sense that both involve splitting a single overall target circuit pattern into two, less dense, target patterns. The two, less dense, target patterns are printed separately using two interlocking patterning masks. For example, a second patterning mask is utilized to print features that interlock with the features patterned by a first patterning mask. SADP has better overlay control than LELE and thus provides a more viable solution for 10 nm route technology and beyond.
In order to create the interlocking first and second patterning masks, design rules must first ensure that the overall target pattern is decomposable. As used herein, the term “decomposable” may be used to refer to the overall target pattern's adherence to various design rules governing the spacing and dimensions of individual device features. Sophisticated electronic design automation (EDA) tools may be used to ensure, for example, that adjacent features are assigned to the same or different patterning mask depending on the distance between the features. The design rules ensure that the overall target pattern is faithfully reproduced by assigning the individual features (based on their spacing and dimensions) to the appropriate patterning mask.
In SADP, a metal wire that is defined by a mandrel pattern is called mandrel metal. A metal wire that is not defined a mandrel pattern is called non-mandrel metal. In design terminology, mandrel metal and non-mandrel metal may be referred to as “different color” metals whereas mandrel metals or non-mandrel metals are respectively referred to as “same color” metals. Design rules enforcing minimum separation distances between metals are applied to mandrel metals and non-mandrel metals separately. For example, mandrel metal to mandrel metal distance is checked by a same color design rule. Similarly, non-mandrel metal to non-mandrel metal distance is checked by a same color design rule. On the other hand, mandrel metal to non-mandrel metal separation distance need be checked by a different color design rule.
SADP imposes restrictions on designers because it requires adherence to a more complex set of design rules. This is especially problematic for dense high-speed cell designs where designers must be able to flexibly design the power routing features. For example, the width of second metal (M2) power rails located adjacent to non-power M2 routes (also referred to as “M2 metal routes” or “M2 layer metal routes”) is greatly restricted for a cell design. For example, in conventional 9-track cell architecture, an M2 power rail that is larger than the default metal width will cost four M2 route tracks due to the restrictive SADP design rule, which causes a significant degradation of chip scaling. Thus, circuit designers are unable to flexibly adjust the width of M2 power rails based on specific electro-migration (EM) and IR drop requirements for a circuit. Although reference will be made to M2 metals, it is contemplated that the disclosure herein is also applicable to other layer metals.
A need therefore exists for methodology for a modified cell architecture that allows flexibly varying the width of metal power rails in double-patterning processes, and the resulting device while maintaining the routing efficiency.